24 
SUPPLEMENT. 
of about 24°, and the line of no declination moved many degrees 
to the westward thereof 
From these observations it is manifest that the Halleyan lines 
in the southern hemisphere gradually move from east to west, 
while the motion of those lines in the northern hemisphere is 
from west to east, and if this conclusion be admitted, viz. that 
the progressive motion of the lines of declination in the northern 
hemisphere be constantly from west to east, this discovery will 
be of as great use to us in framing, regulating, or judging of our 
future charts or tables of the declination or variation of the ma¬ 
riner’s compass, and will answer the purposes of navigation as 
well, as if we were thoroughly acquainted with the primary 
causes of all the phenomena of magnetism. 
It must be allowed, according to the observations of several 
ingenious and scientific men, that the collective magnetism of 
this earth arises from the magnetism of all the ferruginous bodies 
therein contained, and that the magnetic poles should therefore 
be considered as the centres of the powers of those magnetic 
substances. These poles must therefore change their places ac¬ 
cording as the magnetism of such substances is affected : and if 
it be admitted, that the general cause of the diurnal variation 
arises from the sun’s heat in the forenoon and afternoon of the 
same day, it will naturally occur, that the same cause being 
continued may be sufficient to produce the general variation of 
the magnetic needle for any number of years. For it must be 
considered, that, ever since any attentive observations have been 
made on this subject, the natural direction of the magnetic 
needle in Europe has been constantly moving from west to east, 
and that in other parts of the world it has continued its motion 
with equal constancy. 
As we must, therefore, admit that the heat in the different 
seasons depends chiefly on the sun, and upon the whole, that the 
months of July and August will probably be found the hottest, 
while January and February are the coldest months of the year, 
and that the temperature of the other months falls into the respect¬ 
ive intermediate degrees although from calculation we can 
